High-temperature alloy



United States Patent HIGH-TEMPERATURE ALLOY Application February 3, 1954, Serial No. 408,014

6 Claims. (Cl. 75-176) No Drawing.

This invention relates to forgeable high-temperature, high-strength alloys, and more in particular to chromiumiron systems suitable for use in high-strength, high-temperature applications such as are required for turbine buckets in turbojet aircraft engines.

The best present-day, high-temperature alloys from the standpoint of mechanical properties are cobalt-base and nickel-base alloys. However, nickel and cobalt are not available in sufiicient quantities for unlimited use in high-temperature applications.

Chromium is relatively more abundant than either cobalt or nickel, and sufficient quantities would be available for large-scale production of high-temperature, high strength equipment. Various chromium-iron systems have previously been known for good high-temperature strength, but in the past these systems have been characterized bylow ductility, particularly at low temperatures.

It is, the efore, an object of this invention to provide a high-tem erature, high-strength alloy with a minimum of critical materials.

It is also an object of this invention to provide a highstrength, high-temperature, chromium-iron alloy with improved low-temperature ductility.

Another object of this invention is to provide a chromium-iron alloy containing at least 60% chromium, and characterized by high strength and good ductility at high temperatures.

A further object of this invention is to provide an alloy at a reduced cost for use in turbine buckets of turbojet aircraft engines.

Still another object of this invention is to provide .a high-strength, high-temperature, chromium-iron alloy with increased corrosion resistance.

Further objects and advantages of this invention will become apparent from the following specifications and the appended claims.

It has generally been considered that chromium-iron alloys containing greater than 60% chromium are too brittle to be nseable in high-temperature, high-strength applications. In accordance with the present invention, however, it has been found that the use of chromium in greater amounts than 60%, in combination with certain other elements, produces a chromium-iron alloy that has exceptional high-temperature strength and is suf- 2,780,545 Patented Feb. 5, 1957 ice it is not readily available.

In high-chromium alloys of this type, it is necessary to provide some means during melting for eliminating the effects of oxygen and nitrogen in the alloy. This func; tion is here accomplished by small amounts of titanium, which acts, in part, as a nitrogen and oxygen fixer. The amount of titanium required in order to sufficiently fix the nitrogen and oxygen of the alloy is not large, and it has been found that, in the alloys of the present invention, an amount of about 1.4 to 2.0% is sulficient for this purpose. Greater amounts of titanium than 2.0% do not materially affect this function. However, increased titanium content up to about 3.0% has the elfect of enhancing the surface stability of the alloy, and thus increasing its oxidation resistance at high temperatures. Thus, the titanium content of the alloy serves two functions, fixing the oxygen and nitrogen, and enhancing the surface stability. Titanium content greater than 3.0% tends to reduce the forgeability of the alloy. The latter ficiently ductile to have. practical usefulness. The alloy of the present invention has the following composition:

, Percent Chromium -80 Molybdenum 5--11 Titanium 1.4-2 Aluminum and additional titanium upto 1 Iron balance Molybdenum is added to this alloy primarily to impart function may also be accomplished by substituting aluminum or silicon, all or in part, for the titanium content. Thus the composition'of the alloy may be as follows:

. Percent Chromium 60-80 Molybdenum, vanadium, and tungsten 511 Titanium 1 .4-2 Aluminum, silicon, and additional titanium up to 1 Iron balance While in the past it has been indicated that small amounts of titanium in high-chromium alloys improve the stress-rupture characteristics of cast alloys, these additions have not been known to improve the cold malleability of such alloys, and high-chromium alloys in the past have been characterized by extreme brittleness at room temperatures. Because of this lack of 'mall'eability, such alloys were not useful from a practical standpoint. However, in the alloy of the present invention, the addition of titanium has been found to improve the cold malleability to such an extent that the alloy is satisfactory for use in turbine buckets of turbojet aircraft engines.

The additions of titanium to the alloy of the present invention have also been found to improve the forgeability of the alloy. An alloy containing 70 parts of chromium, 30 parts of iron, 3 parts of molybdenum, 3 parts of vanadium, 3 parts of tungsten, and 0.1 part of carbon was found to be forgeable when it contained about 1.75 percent titanium, but cracked on forging when it contained only about 0.85 percent titanium. Similarly, alloys containing 70 parts chromium, 30 parts iron, 4 .parts molybdenum, 4 parts vanadium, 4 parts zirconium, and 0.1 part carbon, and alloys containing 70 parts chromium, 30 parts iron, 4 parts molybdenum, 4 parts vanadium, 4 parts tungsten, and 0.1 part carbon were found to be forgeable when they contained about 1.75 percent titanium, but cracked on forging when they contained only 0.85 percent titanium. The latter two alloys are mentioned here only from the standpoint of showing the effect of titanium in improving forgeability, and they do not possess sufficient strength to be useable.

Special melting techniques are necessary with this alloy in order to reduce the oxygen and nitrogen content of the ingots. Satisfactory results can be obtained when the alloy is melted under a lime-alumina or a lime-alumina-magnesia slag. An argon blanket may be used of'nitrogen and oxygen entering the melt from the air. The slag removes oxygen from the melt as chromium oxides which react with metallic aluminum added to the slag. The. products of this reaction are aluminum oxides, which remain in the slag, and chromium metal' which returns to the melt. The aluminum metal may be added to the slag in any convenient form, such as granules or chips.

The alloys may beheated for forging by heatingthem slowly to 1600 F., holding them at 1600 F for one to to two hours, and then heating them to a forging tem perature of from 2250 to. 2400'F. After forging,.various heat treatments have been. found. to have an unexpected effect upon their highetemperature strength, as will, be seen. These treatments consist in soakingthe alloy after forging for about two hours in an inert atmosphere at a temperature of. from 2200" F.- to 2400 F., and then air cooling. This. may be followed by aging for about 16 hours at about1400 F., although this latter step is not necessary.

Examples of compositions of various high chromiumiron alloys used in tests leading to thepresent invention are given in Table I, wherein the amounts of the constituents are. given in partsby weight and percentages. This table' indicates the intended compositions of the alloys tested. Due to the melting techniques used, the

actual compositions contained somewhat less titanium than indicated. In heats A-8974 to A9368, the titanium recovery was from 60% to 65%. In the remaining heats, the titanium recovery was about 85% to 90%. The aluminum recovery varies from 85% to 90%.

Table I also gives the results ofstress-rupture testat 1500 F. of these alloys. Heat treatment (if any) consisted of two hours soaking at the'specified temperature, and aging (if any) consisted of 16 hours of treatment at the specified temperature. The heat treatment was done subsequent to forging, and those alloys which were not heat treated were tested in the as forged conditiOn.

From these results, it is seen that alloys within the range of the present invention. exhibithigh strength at hightemperatures, and that several of the alloys, e. g.,

intended compositions 70 Cr, 30' Fe, 4 Ti, 9 Mo, and

70 Cr, 30 Fe, 2 Ti, 9 Mo, /2- Al exhibit exceptionally good characteristics. The heat treatment used has insome cases produced a remarkable eifect on the alloys.

These alloys have good high-temperature ductility, and the low-temperature ductility is considerably improved over previous high chromium-iron alloys containing over 60% chromium. Amounts of zirconium up. to 3% may be added to this alloy without materially changing its characteristics;

As is seen in the composition of the alloy system of the present invention, a high-temperature, high-strength alloy is produced without the use of critical materials. The high-temperature ductility isgood, and the low-temperature ductility is satisfactory for use of the alloy in turbine buckets of turbojet aircraft engines. The additives of this alloy have also greatlyincreasedthe high temperature corrosionresistance of chromium-irorrsystems containing over chromium.

Although the alloy system of this invention has been described-as beingspecifically adaptable to use in turbine buckets of turbojet aircraft engines, this application of the alloy system is merely illustrative and is not intended to be a limitation of the invention. The system may be used in any application which requires the use. of the resultant characteristics. of the material without departing fromthe scopetof the. appended claims;

Having; thusdescribed the invention and given' several specific. embodiments thereoflit is believed appar cut that the same may be'widely: varied without essential departure therefrom, andall such modifications and departures are contemplated as may fall-within th'escopc of the following claims.

What isclaimed is:

1. A forgeable high-temperature, high-strength alloyconsisting of60.% to chromium, 5% to 11% molybdenum, 1.4% to 3% titanium, and thebalance essentially 1ron.

2..A' forgeable high-chromium alloy comprising 60% to80% chromium, 5% to 11% molybdenum,-1.4% to 2.0% of titanium asta nitrogenand oxygen-fixing metal,

up to 1.0% of surface-stabilizing metalsof the group consisting of aluminum, silicon and additional titanium,

balance substantially iron, andcharacterized by high strength, high corrosion resistance, and good ductility at high temperatures.

3. An alloy comprising substantially 62% chromium, 8% molybdenum, 2.2% titanium, balance essentially iron, and characterizediby'high strength; high corrosion resistance, and good ductility at. high temperatures.

4. An alloy comprising substantially62.8% chromium, 8.08% molybdenum, 1.7% titanium, and'0.45% aluminum, balanceesscntially iron, and characterized'by high strength,.high. corrosion resistance, and goodfiductility at' high temperatures.

5. A forgeablev high-temperature; high-strength alloy consistingoffrom'60. to 80 percent chromium,.5=to'11' percent molybdenum, 1.4 to 2 percent titanium; up. to 1 percent aluminum, and the balance essentially iron.

Table l 1 (m ositlon-Partsb Per-Cent Stress,v Heat I mended Compos tion Parts by Weight Intende p :7 1,000 Hours N b I v p. s. i. Rupture 01- F6 Ti M6 v W Zr Al Cb 0 Or Fe Ti M6 A] v W- Zr 0b 0 1 0 68.6 27.3 0.0 8. 30 2 6 27.8 1.85 5. 23 s0 2 0 27.1 1.0 8. 23 00 2 3 27 1.8 2. 1 a0 2 a s a 27 1.8 2. 23 55.6 30 2 a a 27.1 1.8 2.7 23 30 4 0 61.0 26.6 3.54 7.96 23 a 4 0 61.0 26.0 3.54 7.06 23 449-7. 60 4 0 61.0 26.6 8.54- 7.96 -9 00 2 12 61.4 26.8 1.75 10.5 5 a0 3i -0 62.5 26.8 2.68 8.03 28 -3 30 2 0 62. 5 26. 8 1. 70 8. 0s 30- 2 0 62.0 27.0 1.79 8.00 28 2 0 62.0 27.0 1.70 8.09 28 1 30 2 0 62.0. 27.0 1.70 8.00 28 A 5054. 30 0 14 62.0 27.0 1.70 8.00 26 14,4 M1523 7 30 2 0 is 62.8 26.0 1.70 8.08 28 357 Heat treatment, Zhours at 2,400degrees 1 1 Heat tr0atn1e11t,'2l1ours at 2,200 degrees F;

6. A forgeable high-temperature, high-strength alloy consisting of from 60 to 80 percent chromium, 5 to 11 percent molybdenum, 1.4 to 2 percent titanium, up to 1 percent silicon, and the balance essentially iron.

References Cited in the file of this patent UNITED STATES PATENTS Sicard July 13, 1920 

1. A FORGEABLE HIGH-TEMPERATURE, HIGH-STRENGTH ALLOY CONSISTING OF 60% TO 80% CHROMIUM, 5% TO 11% MOLYBDENUM, 1.4% TO 3% TITANIUM, AND THE BALANCE ESSENTIALLY IRON. 